you can check it out this code, it might help you...
Dim stringAM As String = "11:30 AM"
Dim stringPM As String = "05:00 PM"
Dim dtAM As DateTime = DateTime.Parse(stringAM)
Dim dtPM As DateTime = DateTime.Parse(stringPM)
If dtPM > dtAM Then
Console.WriteLine("PM is Greter Than AM")
Else
Console.WriteLine("PM is Less Than AM")
End If
Dim stringDTAM As String = "19-March-2014 11:30 AM"
Dim stringDTPM As String = "18-March-2014 05:00 PM"
dtAM = DateTime.Parse(stringDTAM)
dtPM = DateTime.Parse(stringDTPM)
If dtPM > dtAM Then
Console.WriteLine("PM is Greter Than AM")
Else
Console.WriteLine("PM is Less Than AM")
End If
NOTE: if you are trying to check same day time for AM and PM then PM is always greater than AM, but if you are trying to check that with different dates then you might get AM is greater than PM, as shown in above example, in the second part, i have mention PM date is less then AM date, in that case it will return AM is greater then PM